Mechanically Stable Catalyst Based on Alpha-Alumina

ABSTRACT

A catalyst for gas-phase reactions which has high mechanical stability and comprises one or more active metals on a support comprising aluminum oxide as support material, wherein the aluminum oxide in the support consists essentially of alpha-aluminum oxide. 
     Ruthenium, copper and/or gold are preferred as active metal. 
     Particularly preferred catalysts according to invention comprise
     a) from 0.001 to 10% by weight of ruthenium, copper and/or gold,   b) from 0 to 5% by weight of one or more alkaline earth metals,   c) from 0 to 5% by weight of one or more alkali metals,   d) from 0 to 10% by weight of one or more rare earth metals,   e) from 0 to 10% by weight of one or more further metals selected from the group consisting of palladium, platinum, osmium, iridium, silver and rhenium,
 
in each case based on the total weight of the catalyst, on the support comprising alpha-Al 2 O 3 .
   

     The catalysts are preferably used in the oxidation of hydrogen chloride (Deacon reaction).

The invention relates to a mechanically stable catalyst based onalpha-aluminum oxide as support. The invention further relates to such acatalyst for the catalytic oxidation of hydrogen chloride by means ofoxygen to give chlorine and also a process for the catalytic oxidationof hydrogen chloride using the catalyst.

In the process for the catalytic oxidation of hydrogen chloridedeveloped by Deacon in 1868, hydrogen chloride is oxidized by means ofoxygen to give chlorine in an exothermic equilibrium reaction. Theconversion of hydrogen chloride into chlorine enables chlorineproduction to be decoupled from sodium hydroxide production bychloralkali electrolysis. Such decoupling is attractive since theworldwide demand for chlorine is growing more strongly than the demandfor sodium hydroxide. In addition, hydrogen chloride is obtained inlarge amounts as coproduct in, for example, phosgenation reactions, forinstance in isocyanate production.

EP-A 0 743 277 discloses a process for preparing chlorine by catalyticoxidation of hydrogen chloride, in which a ruthenium-comprisingsupported catalyst is used. Here, ruthenium is applied in the form ofruthenium chloride, ruthenium oxychlorides, chlororuthenate complexes,ruthenium hydroxide, ruthenium-amine complexes or in the form of furtherruthenium complexes to the support. The catalyst can comprise palladium,copper, chromium, vanadium, manganese, alkali metals, alkaline earthmetals and rare earth metals as further metals.

According to GB 1,046,313, ruthenium (III) chloride on aluminum oxide isused as catalyst in a process for the catalytic oxidation of hydrogenchloride.

Gamma-aluminum oxide is usually used as aluminum oxide support.

A disadvantage of the known processes which employ catalysts based ongamma-aluminum oxide is the low mechanical strength of the catalysts.This decreases further during use of the catalyst in the reactor. Thelow mechanical strength of the catalysts results in high attrition.Attrition and fine dust formation can lead to overloading of cyclonesand filters or filter chambers in a fluidized-bed process.

It is an object of the present invention to improve the mechanicalstrength of aluminum oxide supports. A further object is to providecatalysts having an increased mechanical strength or gas-phasereactions, in particular for the catalytic oxidation of hydrogenchloride.

This object is achieved by a catalyst for gas-phase reactions which hashigh mechanical stability and comprises one or more active metals on asupport comprising aluminum oxide as support material, wherein thealuminum oxide in the support consists essentially of alpha-aluminumoxide.

It has surprisingly been found that in a support comprisinggamma-aluminum oxide, a phase transformation of gamma-aluminum oxideinto alpha-aluminum oxide occurs in places even at comparatively lowtemperatures as occur, for example, in the gas-phase oxidation ofhydrogen chloride to chlorine (380-400° C.). The domains of crystallinealpha-aluminum oxide formed in this way significantly reduce thestrength of the shaped catalyst body, which is also reflected in thesignificantly increased attrition values of the used catalyst.

The support used according to the invention can comprise alpha-aluminumoxide in admixture with further support materials. Suitable furthersupport materials are, for example, graphite, silicon dioxide, titaniumdioxide and zirconium dioxide, preferably titanium dioxide and zirconiumdioxide, for example in amounts of up to 50% by weight. The supportpreferably consists essentially of aluminum oxide, for example to anextent of 90% by weight and above, and it particularly preferablycomprises at least 96% by weight of aluminum oxide. The aluminum oxidein the support consists essentially of alpha-aluminum oxide, preferablyat least 90% by weight, particularly preferably at least 98% by weight,of alpha-aluminum oxide, based on the total aluminum oxide in thesupport. The phase composition of the support can be determined by XRD(X-ray diffraction).

In general, the catalyst of the invention is used for carrying outgas-phase reactions at a temperature above 200° C., preferably above320° C., particularly preferably above 350° C. However, the reactiontemperature is generally not more than 600° C., preferably not more than500° C.

As active metals, the catalyst of the invention can comprise any activemetals and also further metals as promoters. These are usually comprisedin the catalyst in amounts of up to 10% by weight, based on the weightof the catalyst.

If the catalyst of the invention is to be used in the catalyticoxidation of hydrogen chloride (Deacon process), the active metals areselected from among the elements of groups 7-11 of the Periodic Table ofthe Elements.

Particularly preferred active metals are ruthenium, copper and/or gold.

The supported copper or ruthenium catalysts can be obtained, forexample, by impregnation of the support material with aqueous solutionsof CuCl₂ or RuCl₃ and, if appropriate, a promoter for doping, preferablyin the form of their chlorides. The shaping of the catalyst can becarried out after or preferably before impregnation of the supportmaterial.

Gold-comprising catalysts according to the invention can be obtained byapplication of gold in the form of the aqueous solution of a solublegold compound and subsequent drying or drying and calcination. Gold ispreferably applied as an aqueous solution of AuCl₃ or HAuCl₄ to thesupport.

The ruthenium-, copper- and/or gold-comprising catalysts of theinvention for the catalytic oxidation of hydrogen chloride canadditionally comprise compounds of one or more other noble metalsselected from among palladium, platinum, osmium, iridium, silver andrhenium. The catalysts can also be doped with one or more furthermetals. Suitable promoters for doping are alkali metals such as lithium,sodium, potassium, rubidium and cesium, preferably lithium, sodium andpotassium, particularly preferably potassium, alkaline earth metals suchas magnesium, calcium, strontium and barium, preferably magnesium andcalcium, particularly preferably magnesium, rare earth metals such asscandium, yttrium, lanthanum, cerium, praseodymium and neodymium,preferably scandium, yttrium, lanthanum and cerium, particularlypreferably lanthanum and cerium, or mixtures thereof, also titanium,manganese, molybdenum and tin.

Preferred catalysts according to the invention for the oxidation ofhydrogen chloride comprise

-   a) from 0.001 to 10% by weight, preferably from 1 to 3% by weight,    of ruthenium, copper and/or gold,-   b) from 0 to 5% by weight, preferably from 0 to 3% by weight, of one    or more alkaline earth metals,-   c) from 0 to 5% by weight, preferably from 0 to 3% by weight, of one    or more alkali metals,-   d) from 0 to 10% by weight, preferably from 0 to 3% by weight, of    one or more rare earth metals,-   e) from 0 to 10% by weight, preferably from 0 to 1% by weight, of    one or more further metals selected from the group consisting of    palladium, platinum, osmium, iridium, silver and rhenium,    in each case based on the total weight of the catalyst. The weights    indicated are based on the weight of the metal, even though the    metals are generally present in oxidic form on the support.

Ruthenium is very particularly preferred as active metal and isgenerally comprised in amounts of from 0.001 to 10% by weight, based onthe weight of the catalyst. In a specific embodiment, the catalyst ofthe invention comprises about 1-3% by weight, for example about 1.6% byweight, of ruthenium on alpha-aluminum oxide as support and no furtheractive metals and promoter metals, with ruthenium being present as RuO₂.

The catalysts of the invention are obtained by impregnation of thesupport material with aqueous solutions of salts of the metals. Themetals apart from gold are usually applied as aqueous solutions of theirchlorides, oxychlorides or oxides to the support. Shaping of thecatalyst can be carried out after or preferably before impregnation ofthe support material. The catalysts of the invention are also used asfluidized-bed catalysts in the form of powder having a mean particlesize of 10-200 μm. As fixed-bed catalysts, they are generally used inthe form of shaped catalyst bodies.

The shaped bodies or powders can subsequently be dried at temperaturesof from 100 to 400° C., preferably from 100 to 300° C., for exampleunder a nitrogen, argon or air atmosphere and, if appropriate, calcined.The shaped bodies or powders are preferably firstly dried at from 100 to150° C. and subsequently calcined at from 200 to 400° C. The oxides, forexample RuO₂ or CuO, are performed from the chlorides duringcalcination.

The invention also provides a process for producing catalysts byimpregnating alpha-aluminum oxide as support with one or more metal saltsolutions comprising the active metal or metals and, if appropriate, oneor more promoter metals, and drying and calcining the impregnatedsupport. Shaping to produce shaped catalyst particles can be carried outbefore or after impregnation. The catalyst of the invention can also beused in powder form.

Suitable shaped catalyst bodies include all shapes, with preferencebeing given to pellets, rings, cylinders, stars, wagon wheels orspheres, particularly preferably rings, cylinders or star extrudates.The specific surface area of the alpha-aluminum oxide support prior tometal salt deposition is generally in the range from 0.1 to 10 m²/g.

Alpha-aluminum oxide can be prepared by heating gamma-aluminum oxide totemperatures above 1000° C.; it is preferably prepared in this way. Ingeneral it is calcined for 2-24 hours.

The present invention also provides a process for the catalyticoxidation of hydrogen chloride by means of oxygen to give chlorine overthe catalyst of the invention.

For this purpose, a stream of hydrogen chloride and an oxygen-comprisingstream are fed into an oxidation zone and hydrogen chloride is partiallyoxidized in the presence of the catalyst to give chlorine, so that aproduct gas stream comprising chlorine, unreacted oxygen, unreactedhydrogen chloride and water vapor is obtained. The hydrogen chloridesteam, which can originate from a plant for the preparation ofisocyanates, can comprise impurities such as phosgene and carbonmonoxide.

The reaction temperatures are usually in the range from 150 to 500° C.,and the reaction pressures are usually in the range from 1 to 25 bar,for example 4 bar. The reaction temperature is preferably >300° C.,particularly preferably from 350° C. to 400° C. Furthermore, it isadvantageous to use oxygen in superstoichiometric amounts. A 1.5- tofour-fold, for example, oxygen excess is usual. Since no losses ofselectivity have to be feared, it can be economically advantageous tocarry out the reaction at relatively high pressures and accordingly atresidence times which are longer than under atmospheric pressure.

Usual reaction apparatuses in which the catalytic oxidation according tothe invention of hydrogen chloride is carried out are fixed-bed orfluidized-bed reactors. The oxidation of hydrogen chloride can becarried out in one or more stages.

The catalyst bed or fluidized catalyst bed can comprise further suitablecatalysts or additional inert material in addition to the catalyst ofthe invention.

The catalytic oxidation of hydrogen chloride can be carried outadiabatically or preferably isothermally or approximately isothermally,batchwise or preferably continuously as a moving-bed or fixed-bedprocess, preferably as fixed-bed process, particularly preferably inshell-and-tube reactors, at reactor temperatures of from 200 to 500° C.,preferably from 300 to 400° C., and a pressure of from 1 to 25 bar,preferably from 1 to 5 bar.

In isothermal or approximately isothermal operation, it is also possibleto use a plurality of reactors, for example from 2 to 10 reactors,preferably from 2 to 6 reactors, particularly preferably from 2 to 5reactors, in particular 2 or 3 reactors, connected in series withadditional intermediate cooling. The oxygen can either all be addedtogether with the hydrogen chloride upstream of the first reactor ordistributed over the various reactors. This series arrangement ofindividual reactors can also be combined in one apparatus.

In one embodiment of the fixed-bed process, a structured catalyst bed inwhich the catalyst activity increases in the direction of flow is used.Such structuring of the catalyst bed can be achieved by differentimpregnation of the catalyst support with active composition or bydifferent dilution of the catalyst bed with an inert material. Inertmaterials which can be used are, for example, rings, cylinders orspheres comprising titanium dioxide, zirconium dioxide or mixturesthereof, aluminum oxide, steatite, ceramic, glass, graphite or stainlesssteel. The inert material preferably has similar external dimensions tothe shaped catalyst bodies.

The conversion of hydrogen chloride in a single pass can be limited tofrom 15 to 90%, preferably from 40 to 85%. Unreacted hydrogen chloridecan, after having been separated off, be recirculated in part or in itsentirety to the catalytic oxidation of hydrogen chloride. The volumeratio of hydrogen chloride to oxygen at the reactor inlet is generallyfrom 1:1 to 20:1, preferably from 1.5:1 to 8:1, particularly preferablyfrom 1.5:1 to 5:1.

The chlorine formed can subsequently be separated off in a customarymanner from the product gas stream obtained in the catalytic oxidationof hydrogen chloride. The separation usually comprises a plurality ofstages, namely isolation and, if appropriate, recirculation of unreactedhydrogen chloride from the product gas stream from the catalyticoxidation of hydrogen chloride, drying of the resulting residual gasstream which consists essentially of chlorine and oxygen and separationof chlorine from the dried stream.

The invention is illustrated by the following examples.

EXAMPLES Determination of Attrition and Proportion of Fines by theMontecatini Method

The attrition test simulates the mechanical stresses to which afluidized material (e.g. a catalyst) is subjected in a fluidizedgas/solid bed and gives as results an attrition value (AT) and aproportion of fines (PF) which describe the strength behavior.

The attrition apparatus comprises a nozzle plate (nozzle diameter=0.5mm) which is connected in a gastight and solids-tight manner to a glasstube. A steel tube having a conical widening is fixed above the glasstube, likewise in a gastight and solids-tight manner. The apparatus isconnected to the 4 bar compressed air supply. A reducing valve decreasesthe pressure to 2 bar absolute upstream of the apparatus.

60.0 g of catalyst are introduced into the apparatus. The amount ofcompressed air for carrying out the experiment is 350 l/h. The apparatusitself is operated under atmospheric conditions (1 bar, 20° C.). Owingto the high gas velocity at the nozzle, the particles are subjected toabrasion or broken up by particle/particle and particle/wall impacts.The solid discharged travels via a tube bend into a filter paper thimble(pore size: 10-15 μm) and the purified gas flows into the exhaust airsystem of the laboratory.

The solid which has been deposited is weighed after one hour (defined asthe proportion of fines PF) and after 5 hours (defined as the attritionAT).

Example 1

A pulverulent gamma-aluminum oxide support from Sasol (Puralox® SCCa30/170) was firstly converted into alpha-Al₂O₃. The support consists ofparticles having a mean particle diameter of about 50 μm. For thispurpose, 2000 g of Puralox® SCCa 30/170 were heated at 1200-1300° C. for5 hours. 1500 g of the support obtained were impregnated with an aqueousRuCl₃ hydrate solution (55.56 g of RuCl₃ hydrate corresponding to 41.8%by weight of Ru in 480 g of water). The water uptake of the support wasabout 0.38 ml/g. After impregnation to 90% of the water uptake, theimpregnated support was dried at 120° C. for 6 hours and subsequentlycalcined at 350° C. for 2 hours. The catalyst produced in this waycomprises about 2% of RuO₂ on alpha-Al₂O₃. The most important propertiesof the catalyst are summarized in Table 1.

Comparative Example C1

The gamma-aluminum oxide support Puralox® SCCa 30/170 was used directlyfor producing the catalyst without prior heat treatment. About 1434 g ofthe support were impregnated with an aqueous RuCl₃ hydrate solution(54.1 g of RuCl₃ hydrate corresponding to 36.5% of Ru in 1045 g ofwater). The water uptake of the support was about 0.81 ml/g. The supportwhich had been impregnated to 90% of the water uptake was dried at 120°C. for 6 hours and calcined at 350° C. for 2 hours. The catalystproduced in this way comprises about 2% of RuO₂ on gamma-Al₂O₃. The mostimportant properties of the catalyst are summarized in Table 1.

TABLE 1 Attrition by Proportion of the fines by the Phase RuO₂ contentBET Montecatini Montecatini composition [% by surface area method methoddetermined by Example weight] [m²/g] [g/60 g] [g/60 g] XRD 1 1.98 4 3.61.4 alpha-Al₂O₃ C1 2.10 163 8.4 2.8 gamma-Al₂O₃

Example 2 and Comparative Example 2

A Deacon reactor was operated in the fluidized-bed mode using thecatalysts from Example 1 and Comparative Example C1. The Deacon reactorconsisted of a tube having a diameter of 4 cm and a length of 1 m andcomprised 600 g of the catalyst. At 380-400° C. and a reactor pressureof 4 bar, 200 standard l/h of HCl and 100 standard l/h of O₂ were fedinto the reactor; the HCl conversion was 60-80%. After operation of thereactor for 1000 hours, the catalyst was removed. The catalystproperties of the used catalyst are summarized in Table 2.

TABLE 2 Attrition by Proportion of the fines by the Phase RuO₂ contentBET Montecatini Montecatini composition [% by surface area method methoddetermined by Example weight] [m²/g] [g/60 g] [g/60 g] XRD 1 2.0 2.8 3.81.2 alpha-Al₂O₃ C1 1.9 120 20.5 16 gamma-Al₂O₃ + alpha-Al₂O₃

As can clearly be seen from the values for proportion of fines andattrition, the catalyst according to the invention has a significantlyhigher mechanical stability compared to the corresponding catalyst ongamma-aluminum oxide as support. This is the case even for the freshlyproduced catalyst, but in particular for the used catalyst.

1-11. (canceled) 12: A fluidized bed catalyst for gas-phase reactionswhich has high mechanical stability in the form of powder having a meanparticle size of from 10 to 200 μm and comprises one or more activemetals on a support consisting essentially of aluminum oxide as supportmaterial, wherein the aluminum oxide in the support consists essentiallyof alpha-aluminum oxide. 13: The catalyst according to claim 12, whereinthe active metal or metals is/are selected from among the elements ofgroups 7-11 of the Periodic Table of the Elements. 14: The catalystaccording to claim 12, wherein the active metal is ruthenium, copperand/or gold. 15: The catalyst according to claim 12 comprising a) from0.001 to 10% by weight of ruthenium, copper and/or gold, b) from 0 to 5%by weight of one or more alkaline earth metals, c) from 0 to 5% byweight of one or more alkali metals, d) from 0 to 10% by weight of oneor more rare earth metals, e) from 0 to 10% by weight of one or morefurther metals selected from the group consisting of palladium,platinum, osmium, iridium, silver and rhenium, in each case based on thetotal weight of the catalyst. 16: The catalyst according to claim 12,wherein the active metal is ruthenium. 17: A process for producing afluidized bed catalyst according to claim 12 by impregnatingalpha-aluminum oxide in the form of powder having a mean particle sizeof from 10 to 200 μm as support with one or more metal salt solutionscomprising the active metal or metals and, if appropriate, one or morepromoter metals, and drying and calcining the impregnated support. 18:The process according to claim 17, wherein alpha-aluminum oxide isprepared by heating gamma-aluminum oxide to temperatures above 1000° C.19: A process for the catalytic oxidation of hydrogen chloride by meansof oxygen to give chlorine over a fluidized catalyst bed comprisingcatalyst particles of the catalyst according to any of claim
 12. 20: Theprocess according to claim 18, wherein the catalytic oxidation iscarried out at a reaction temperature of >350° C.